Electrical power converters are devices for processing electrical power from one form, such as a positive DC voltage, into another form, such as one or more DC output voltages. It is common for electrical power converters to provide output power at two or more output ports each having a different output voltage level, commonly referred to as a "voltage rail." Such multiple-output power converters often have at least one output voltage rail which is negative with respect to a ground reference voltage.
For many applications, it is important to regulate the output voltages of an electrical power converter to predetermined values. In such cases, the output voltages of the power converter are commonly controlled by a regulator circuit, such as a switching regulator in the case of switch mode power converters. A switching regulator commonly controls one or more switches that govern the coupling of voltage and current from the input of the converter to a power transformer within the power converter. The regulation process is commonly performed by a pulse-width modulator that is responsive to at least one output voltage of the power converter. By varying the duration of the voltage and current pulses coupled through the switches, the pulse-width modulator controls the output voltages of the converter.
Conventional pulse-width modulator circuits commonly are capable of directly detecting and regulating only positive output voltages. Negative output voltages are generally controlled by virtue of being functionally related to a, positive output voltage which is directly regulated. Alternatively, a negative output voltage may be inverted into a positive voltage, which is then detected and regulated by a pulse-width modulator such that the negative output voltage rail is regulated.
In circuits where the negative output rail is controlled by virtue of being functionally related to a regulated positive output rail, there are often problems with cross-regulation. For example, an increase in the load current on the positive output results in the pulse-width modulator increasing the pulse width to keep the voltage of the positive output rail constant. Increasing the pulse width, however, also increases the magnitude of the negative output rail voltage, which may result in a lower negative output voltage than desired.
Where the negative output voltage is converted to a positive output voltage in prior art systems, a conventional operational amplifier, or "op-amp", is commonly used so that the negative rail can be regulated by a conventional pulse-width modulator. The advantage of using an op-amp circuit for this purpose is its precision. Using op-amps for this purpose, however, has limitations. Op-amps are relatively costly. Additionally, op-amps typically require resistors to perform the desired detection function. The op-amp and associated resistors occupy a significant amount of area on a printed circuit board ("PCB").
As an alternative to regulating only one output voltage, the power converter may contain separate pulse width modulators and separate primary power transformer windings for producing each of the regulated output voltages. In such cases, the pulse width modulators are commonly positioned on the primary side of the power transformer. Each output voltage is then measured in a conventional manner using an opto-isolator to transmit a measurement signal to the corresponding pulse width modulator. The opto-isolator additionally serves to maintain isolation between the primary and secondary sides of the power converter. This approach has the following disadvantages. First, more than one pulse width modulator and primary power transformer winding are necessary to regulate multiple output voltages. Additionally, a separate output voltage detection circuit is required for each regulated output voltage, with the corresponding optooisolators and resistors for each circuit. Third, additional circuit elements increase costs and printed circuit board area requirements.
Accordingly, there is a need for a relatively efficient and inexpensive circuit for detecting one or more negative output voltage rails of an electrical power converter and providing a signal that can be used by conventional pulse-width modulators for regulating the negative output voltage. There is a further need for a relatively inexpensive output voltage detection circuit for use in regulating more than one output voltage such that cross-regulation problems are reduced.